ISBN: 978-83-60043-10-3 - eurobic9

Eurobic9, 2-6 September, 2008, Wrocław, Poland
O20. Biosynthetic Exchange of Bromide for Chloride and Strontium for
Calcium in the Photosystem-II Oxygen Evolving Complex
N. Ishida a , M. Sugiura b , F. Rappaport c , T.-L. Lai a , A.W. Rutherford a , A. Boussac a
a
DSV, iBiTec-S, URA CNRS 2096, CEA Saclay, 91191, GIf-sur-Yvette, France
e-mail: alain.boussac@cea.fr
b
Department of Plant Biosciences, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, 599-8531, Sakai,
Osaka, Japan
c
Université Pierre et Marie Curie, Institut de Biologie Physico-Chimique, CNRS UMR 71, 13 rue Pierre et
Marie Curie, 75005, Paris, France
Light-driven water oxidation by Photosystem II (PSII) is responsible for the O2 on Earth and most of the
biomass. Refined 3D X-ray structures at 3.5 Å and at 3.0 Å resolution have been obtained by using PSII isolated
from the thermophilic cyanobacterium Thermosynechococcus elongatus [1, 2]. The active site for water
oxidation in PSII goes through five sequential oxidation states before O2 is evolved. It consists of a Mn4Cacluster
close to a redox-active tyrosine residue and possibly Cl - as cofactor. To study the role of Ca 2+ and Cl - ,
T. elongatus was grown in the presence of Sr 2+ instead of Ca 2+ and Br - instead of Cl - , in order to biosynthetically
substitute the Ca 2+ and Cl - for Sr 2+ and Br - , respectively. Irrespective of the combination of the non-native ions
used (Ca/Br, Sr/Cl, Sr/Br), the PSII could be isolated in a state that was fully intact but kinetically limited. The
step(s) of the enzyme mechanism affected by the exchanges were identified then investigated by using timeresolved
UV-visible absorption spectroscopy, time-resolved O2 polarography, thermoluminescence and EPR
spectroscopy. The effect of the Ca/Sr and Cl/Br exchanges were additive and the magnitude of the effects varied
with the following order: Ca/Cl < Ca/Br < Sr/Cl < Sr/Br. All the observations indicate that Cl - is involved in the
water oxidation mechanism. If so, the lack of a Cl - binding site in the current 3D models of the enzyme from
X-ray crystallography may be ascribable to insufficient resolution.
References:
[1] Ferreira, K. N., Iverson, T. M., Maghlaoui, K., Barber, J., and Iwata, S. (2004) Science 303, 1831-1838.
[2] Loll, B., Kern, J., Saenger, W., Zouni, A., and Biesiadka, J. (2005) Nature 438, 1040-1044.
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